In recent years a solar cell (solar power generation) has been under intensive studies in view of sustainability, no involvement in the resource depletion problem, and environmental friendliness. Solar cells are roughly divided into inorganic solar cells including silicone-based and non-silicone-based solar cells and organic solar cells including dye-sensitized solar cells and organic thin film solar cells. Although inorganic solar cells generally achieve high photoelectric efficiency, they have the disadvantage of high production cost because of the necessity of high degree of vacuum and high-temperature heat treatment. Because organic solar cells, on the other hand, are fabricated by film formation by a solution process, or printing, they can be produced at a lower cost and with a larger effective area. Another advantage of organic solar cells is that they can be made lighter than inorganic solar cells. In particular, an organic thin-film solar cell is suitably fabricated by a printing technique that is applicable to a plastic film substrate and is regarded as easily capable of taking on the form of a flexible device.
However, organic solar cells often have low photoelectric efficiency, and the challenge is to increase the photoelectric efficiency.
A bulk heterojunction formed of a blend of poly(3-hexylthiophene) (P3HT) as a p-type organic semiconductor material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an n-type organic semiconductor material has been so far proposed as a material that can achieve high photoelectric efficiency in organic thin-film solar cells (see non-patent Literature 1, etc. listed below). While a low-molecular compound, such as pentacene, is used in some cases, a polymer material is generally considered as suited to fabricate a solution-processed device, and it is believed that using a polymer material as a p-type organic semiconductor material makes it easier to hold down the cost and increase the effective area.
One of the characteristics demanded for a p-type organic semiconductor material is to contain a pi-conjugated system with high planarity. To have a highly planar pi-conjugated system in the material promises to benefit a strong pi-pi interaction and a high carrier transport efficiency and, as a result, provide high photovoltaic power.
Patent Literatures 1 to 3 below disclose techniques relating to p-type organic polymer semiconductor materials.